The present invention relates broadly to alignment fixtures and jigs for precisely positioning printed circuit board (PCB) workpiece during a manufacturing operation, and more specifically to fixtures and jigs having a pair of locating pins for insertion in a pair of holes in the workpiece.
During manufacture of a workpiece, it often is necessary to precisely position the workpiece in an alignment fixture or jig. The location of the workpiece in the fixture defines a frame of reference for establishing the location at which various machining or other manufacturing operations are to be performed.
In order to layout a hole pattern on a printed circuit board (PCB) or other similar assemblies an accurate datum line is required. The datum line is used as a reference point from where all other dimensions are referenced. Currently, a datum line is established by drilling two close tolerance pins located on a base plate. Because of the tolerances required the establishment of a datum line is both difficult and costly.
The task of providing an alternative to the practice of using an accurate datum line with a tight tolerance, is all alleviated to some extent by the system disclosed in the following U.S. patents, the disclosures of which specifically incorporates by reference:
U.S. Pat. No. 3,392,253 issued to Adams; PA0 U.S. Pat. No. 3,713,944 issued to Dennis; and PA0 U.S. Pat. No. 4,058,890 issued to Pierce.
All of the above-cited references are examples of prior art. Additionally, some conventional fixture designs are described in the "Tool Engineers Handbook", F. W. Wilson, editor, 1959.
One of the most basic fixture designs comprises the use of two cylindrical pins which are intended to be inserted in two corresponding circular holes, shown as locating holes, in the workpiece. A shortcoming of this design is (as described above) that the two locating hole must be manufactured with extreme accuracy in their diameters and in the spacing between them. If, due to manufacturing variations, the spacing between the two locating holes is slightly off or the diameter of one of the two holes is slightly smaller than that of its corresponding pin, it will be difficult or impossible to push the pins through the holes, thereby preventing the workpiece from being mounted on the fixture. On the other hand, if the diameter of one of the holes is significantly larger than that of its corresponding pin, the workpiece will be able to slip relative to the pin and will not be precisely located on the fixture.
A known alternative to the foregoing fixture design using two cylindrical pins is to use one cylindrical pin and one "diamond" pin. In the "Tool Engineers Handbook" cited above, this design is illustrated in FIGS. 91-5 and 91-9 and is discussed in the third paragraph on page 91-8 and in the text beginning with the last paragraph on Page 91-9. The diamond pin is shaped so as to contact its corresponding locating hole only at two small accurate contact surfaces located at either end of a diameter of the diamond pin, the diameter being perpendicular to a line drawn between the two pins. The diamond pin shape provides clearance around the pin for escape of unwanted chips and dirt, and the shape also eases insertion and removal of the pin when the spacing between the two holes is very slightly off.
However, the fixture using the diamond pin does not overcome the shortcoming of the previously discussed fixture in accommodating variations in the spacing between, and the diameters of the two locating holes in the workpiece. Both fixture designs prevent insertion of the locating pins if either of the locating holes is too small or if the spacing between the holes is significantly off, and both designs allow slippage of the workpiece if either of the locating holes is too big.
Another type of conventional fixture overcomes some of the problems of the foregoing designs by using a spring-loaded conical locating pin. This type of fixture is disclosed in FIG. 91-11 of the cited "Tool Engineers Handbook" and in the text referring to that figure at pages 91-10 and 91-13. Such a fixture accurately centers a locating hole of the workpiece about the pin, independent of variations in the hole diameter, because the spring pushes the conical pin into the hole to the depth at which the diameter of the pin equals that of the hole.
Although conventional fixtures have used a spring-loaded conical pin to accommodate variations in the diameter of a locating hole in a workpiece, no known fixture accommodates substantial variations in the spacing between two locating holes. For example, suppose a fixture has two spring-loaded conical pins designed to be inserted in two locating holes in a workpiece. If the spacing between the holes is inaccurate, the two holes cannot both be centered over their corresponding pins, and there will be no uniquely determined alignment of the workpiece on the fixture. For this reason, spring-loaded conical pins are usually used singly to center a workpiece, but not in pairs to precisely locate a workpiece in two or three dimensions.